1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a direct-type backlight unit including a plurality of light emitting diodes and a liquid crystal display device having the same.
2. Discussion of the Related Art
Cathode ray tubes (CRTs) are one of the commonly used display devices. The CRTs have been used for monitors of various electric devices such as televisions (TVs), meters, computers and intelligent terminals. Recently, these electronic devices have become lighter and thinner to satisfy users demand. However, there is limitation in reducing the size and weight of these electronic devices due to the size and weight of the CRTs.
To overcome such a limitation, several types of display devices have been introduced such as liquid crystal display (LCD) devices that display images using an electro-optical effect, plasma display panels (PDPs) that display images using a gas discharge, and electro-luminescence display (ELDs) that display images using an electro-luminescence effect. Among these display devices, LCD devices have been most actively studied and developed. LCD devices are now widely used for monitors of laptop computers, desktop computers and information display devices.
In general, a liquid crystal display (LCD) device displays images by controlling an amount of light emitted from an external light source. Therefore, the LCD device requires an additional light source such as a backlight unit for emitting light to a liquid crystal panel. The backlight unit generally includes a plurality of cylindrical light emitting lamps and is classified into two types: the edge-type backlight unit and the direct-type backlight unit.
In a dual-edge-type backlight unit, a lamp unit is arranged at one edge of a light guide panel. The lamp unit includes a lamp for emitting light, a lamp holder formed at each end of the lamp for protecting the lamp and a lamp reflector plate for reflecting light emitted from the lamp toward the light guide panel. The lamp reflector plate covers an outer periphery of the lamp and one side of the lamp reflector plate is connected to a side edge of the light guide panel. Such an edge-type backlight unit is commonly used in small-size LCD devices such as monitors of laptop or desktop computers.
The direct-type backlight unit has been developed for large-size display devices (i.e., greater than twenty inches). In the direct-type backlight unit, a plurality of lamps are arranged in a row and a diffusion plate is arranged on the plurality of lamps. The lamps emit light directly to the entire surface of the liquid crystal panel. Since the direct-type backlight unit efficiently uses the light emitted from the lamps compared to the edge-type backlight unit, it is mainly used in large-size LCD devices such as large-size monitors or televisions (TVs) which require a high luminance. Compared with a backlight unit for small-size LCD devices, the direct-type backlight unit for large-size LCD devices is more susceptible to short lifetime or breakdown of the lamps and uses a larger number of lamps.
In case of the edge-type backlight unit where the lamp unit is arranged at both ends of the light guide panel, even when one of the lamps breaks down, the LCD device can display images without a major problem, although the luminance of the LCD device is degraded. However, in case of the direct-type backlight unit where a plurality of lamps are arranged under the liquid crystal panel, when one of the lamps breaks down, the display area corresponding to the malfunction lamp becomes noticeably darkened. Accordingly, the lamps of the LCD device using the direct-type backlight unit should be frequently replaced with new lamps. Thus, it should be easy to assemble and disassemble the LCD device having the direct-type backlight unit. The edge-type and direct-type backlight units suffer from low efficiency of light usage caused by assembling and disassembling the lamps and heat generated from the lamps.
Hereinafter, a backlight unit according to the related art will be explained with reference to the accompanying drawings. FIG. 1 is a perspective view of a direct-type backlight unit according to the related art.
Referring to FIG. 1, the backlight unit includes a plurality of light emitting lamps 1, an outer case 3 for supporting and fixing the light emitting lamps 1 and a light diffusion unit 5a, 5b, 5c arranged between the light emitting lamps 1 and a liquid crystal panel (not shown).
The light diffusion units 5a, 5b, 5c provide a uniform luminance to the liquid crystal panel and prevent a shape of the light emitting lamps from being shown on a display side of the liquid crystal panel. Accordingly, a plurality of diffusion sheets and diffusion plates may be provided between the liquid crystal panel and the backlight unit in order to improve light diffusion efficiency.
A reflector plate 7 is arranged inside the outer case 3 for focusing the light emitted from the light emitting lamps 1 to the display side of the liquid crystal panel. That is, the reflector plate 7 maximizes the efficiency of light usage.
Cold cathode fluorescent lamps (CCFL) are used as the light emitting lamps 1. The light emitting lamp 1 includes two electrodes at each end of the tube. When electric power is supplied to the electrodes, the light emitting lamp 1 emits light. The light emitting lamp 1 is placed in a groove formed on both sides of the outer case 3. The electrodes of the light emitting lamp 1 are connected to power input lines 9a, 9b to supply electric power to the light emitting lamp and the power input lines 9a, 9b are connected to an additional connector connected to an operating circuit. Accordingly, an additional connector is required for each light emitting lamp 1. That is, the power input lines 9a, 9b connected to the electrodes are connected to a single connector and one of power input lines 9a, 9b is bent under the outer case 3 and connected to the connector.
External electrode fluorescent lamps (EEFL) may be used as the light emitting lamps instead of using the CCFL. The EEFL includes electrodes located outside each end of the tube.
The direct-type backlight unit according to the related art has the following problems. First, heat generated from the light emitting lamps is directly transferred to a LCD module including the LCD panel because the light emitting lamps are included inside the LCD module as a light source. Accordingly, the LCD device including the direct-type backlight unit generates a comparatively large amount of heat that degrades the performance, especially when the LCD device is used in such environments as vehicles or military environments. Second, because the direct-type backlight unit includes a plurality of light emitting lamps, especially when it is employed for large-size display devices, the LCD device including the direct-type backlight unit consumes a large amount of power. Third, when the direct-type backlight unit includes a gas such as mercury (Hg), it suffers from low durability and may also create an environmental problem.
Hereinafter, a direct-type backlight unit having a plurality of light emitting diodes (LEDs) according to the related art will be described. The direct-type backlight unit having LEDs may be called a LED-direct-type backlight unit. FIG. 2 is a perspective view illustrating a direct-type backlight unit having a plurality of light emitting diodes (LEDs) according to the related art.
Referring to FIG. 2, the LED-direct-type backlight unit includes a printed circuit board (PCB) 42, a plurality of light emitting diodes (LEDs) 44 formed on the PCB 42 for emitting light, and a diffusion plate 46 for uniformly diffusing the light emitted from the LEDs 44.
Each of the LEDs 44 includes a lens formed of a plastic material. The lens is transparent and does not include a pattern for diffusing light. Each of the LEDs 44 is a point light source and emits a red light, a green light or a blue light. A circuit for controlling the LEDs 44 is formed on the PCB 42. The PCB 42 also supports the LEDs 44 and controls the LEDs 44 using the circuit formed on the PCB 42.
A reflector plate is formed over the lens of the LEDs 44 for diffusing the light emitted from the LEDs 44 in a lateral direction. Accordingly, red, green and blue lights are mixed inside a bottom cover (not shown) and thus a white light is generated and directed toward a liquid crystal panel (not shown). The diffusion plate 46 is provided within a predetermined space for diffusing the white light emitted from the LEDs 44 (uniform distribution).
FIG. 3 is a schematic diagram illustrating a LED lamp unit of the LED-direct-type backlight unit illustrated in FIG. 2. Referring to FIG. 3, lighting emitting diodes (LED) emitting a red light R, a green light G and a blue light B are arranged in order to display the corresponding colors. The LEDs may be turned on frame by frame or all LEDs may be simultaneously turned on for generating a white light which is then directed toward the liquid crystal panel.
Although the LED-direct-type backlight unit includes the diffusion plate 46 over the lens of the LEDs 44, about 10% of the light is not mixed and propagates in a vertical direction, thereby generating a red, green or blue spot on the LCD screen. Such a spot degrades the quality of the images displayed on the LCD screen. Thus, the backlight unit and the liquid crystal panel should be separated by a predetermined space, thereby increasing the size of the LCD device.